The present invention relates to an accumulator fuel injection apparatus.
An accumulator fuel injection apparatus, generally known as a common rail type fuel injection apparatus, is preferably used to inject fuel into a diesel engine. According to the accumulator fuel injection apparatus, a common accumulator piping (i.e., common rail) is provided to supply fuel to each cylinder of the engine. A supply pump is provided to supply pressurized fuel into this common rail so that a hydraulic pressure of the fuel in the common rail is maintained at a predetermined level. The accumulated fuel of the common rail is introduced into each fuel injector via a fuel supply pipe.
The accumulated fuel supplied to the fuel injector is chiefly injected into a combustion chamber of each cylinder. However, part of the accumulated fuel is used to control the fuel injector. This kind of control fuel is introduced into a control chamber. An electromagnetic valve opens or closes a fuel discharge passage of the control chamber to adjust a hydraulic pressure of the control chamber. The control chamber controls opening and closing of a needle valve that determines injection and shutoff periods of the fuel injector. The electromagnetic valve discharges the fuel from its valve opening to a lowpressure return passage via a switching leak passage. Furthermore, when the fuel leaks from any slide portion of the fuel injector, the fuel returns via a stationary passage to the low-pressure return passage.
The electromagnetic valve has an armature driven by a solenoid to open-and-close control the valve opening of the electromagnetic valve. The armature is accommodated in an armature chamber. This armature chamber is filled with the fuel to stabilize the operation of the armature. As an arrangement for introducing the fuel into the armature chamber, the armature chamber may be located downstream of the valve opening of the electromagnetic valve in the switching leak passage, as disclosed in the published Japanese Patent Application No. Kokai 9-42106, corresponding to the U.S. Pat. No. application Ser. No. 08/686,774.
However, when incorporated into recent advanced engines, the above-described fuel injection apparatus cannot satisfy various requirements for realizing precise engine controls. More specifically, a great amount of bubbles are generated in the vicinity of a valve opening of the electromagnetic valve when the hydraulic pressure of the accumulated fuel in the control chamber abruptly reduces to a lower value in response to the valve opening operation. The generated bubbles enter the armature chamber. When the armature chamber is filled with bubble-containing fuel, the armature does not operate stably. Furthermore, the fuel leak amount varies depending on engine operating conditions, causing changes in the hydraulic pressure of the armature chamber and in the bubble amount so that the operation varies in a complicated manner. When realizing the precise engine controls, such unstable operation of the armature (i.e., open and close control of the electromagnetic valve) will cause various problems including fluctuation of the fuel injection amount with respect to a set value.
According to another conventional method of introducing the fuel into the armature chamber, it is possible to form a dead alley branching from the switching leak passage at a portion just downstream of a valve opening of the electromagnetic valve. The armature chamber is provided at the dead end of this alley so as to prevent bubbles generated at the valve opening of the electromagnetic valve from directly entering the armature chamber. However, this arrangement is disadvantageous in that air may enter the armature chamber during installation and the residual air in the armature chamber cannot be discharged easily. This sensitively changes environment of the armature depending on the engine operating conditions. The armature is soaked in the fuel in some cases and exposed to the air in other cases. This is not preferable in realizing accurate engine control.
Furthermore, according to the above-described accumulator fuel injection apparatus, when the needle valve is closed, a hydraulic pressure of the armature chamber changes abruptly. Operation of the electromagnetic valve is not stabilized. FIG. 14 shows a variation of a valve lift amount relative to elapse of time. Due to unstable operation of the electromagnetic valve, the needle valve causes a large bounce after the needle valve is once seated to stop the fuel supply. Such a bouncing behavior of the needle valve causes a significant delay in the shutoff operation of the fuel injection. As a result, an actual fuel injection amount exceeds a set value predetermined based on engine operating conditions, such as engine load or the like. The valve bouncing behavior is not constant and variable depending on engine operating conditions as well as individual differences of needle valves. Accordingly, as a matter of practical problem, correcting the error caused between the actual fuel injection amount and the set value is difficult. The engine controls cannot be accurately performed.